In a storage phosphor imaging system as described in U.S. Pat. Re. No. 31,847, reissued Mar. 12, 1985, inventor Luckey, a storage phosphor is exposed to an x-ray image of an object, such as the body part of a patient, to record a latent x-ray image in the storage phosphor. The latent x-ray image is read out by scanning the storage phosphor in a raster pattern with relatively long wavelength stimulating radiation (such as red or infrared light produced by a gas or diode laser). Upon stimulation, the storage phosphor releases emitted radiation of an intermediate wavelength, such as blue light, in proportion to the quantity of x-rays received. The emitted radiation from the storage phosphor is reflected by a mirror light collector and detected by a photodetector which can include one or more photomultiplier tubes, to produce an x-ray image signal. The x-ray image signal may then be processed for tone scale and edge enhancement.
In order to be able to reuse the storage phosphor, any residual image is erased by exposing the storage phosphor to an erase light source. Balanced with the need to erase all of the residual image from the storage phosphor is the desire to increase productivity of the storage phosphor reader. In a known storage phosphor reader, the erase time is fixed at between 10 to 30 seconds (depending on the storage phosphor size) which is adequate to reduce the latent image of a 200 mR x-ray exposure to a residual image of less than 0.01 mR (residual fraction of &lt;5.times.10-5). This residual image would then be undetectable in a subsequent exposure and scan. It is also known that a storage phosphor exposure of 200 mR is uncommon in diagnostic radiology. This amount of plate exposure translates into an entrance skin exposure of approximately 350 mR for average thickness body habitus at a typical x-ray source to image distance (SID). Also, in the case of grids used during exposure, a Bucky factor (i.e., the ratio of incident x-ray radiation to transmitted x-ray radiation passing through a lead grid) of 2-5 is common for the range of grid ratios and incident x-ray spectra used most typically in diagnostic radiography techniques. The Bucky factor, therefore, increases the potential entrance skin exposure to the range of 700-1500 mR for a 200 mR exposure incident on the storage phosphor.
Because of the potential biological hazards of ionizing radiation, such as diagnostic x-rays, an order of magnitude or less plate exposure is typically seen in diagnostic x-ray procedures. (See, e.g., the Common Entrance Skin Exposures for Various Radiographic Examinations, Table 1, FIG. 7). As shown in FIG. 7, the approximate plate exposure ranges from 10 to 145 mR. The approximate plate exposure is calculated from a scaling due to average patient thickness (scale factor=(SOD/SID).sup.2) and a scaling over a reasonable range of Bucky factors (2 to 5).
Since the time required to erase a storage phosphor is proportional to the x-ray exposure it receives, the machine cycle time is, unfortunately, maximized by fixing erase time to handle the worst case x-ray exposure scenario, and consequently the productivity of the reader is reduced. Erasing, for example, a 20 mR phosphor x-ray exposure requires substantially less time than 10 seconds for all plate sizes. Thus there is a need to increase productivity in storage phosphor readers. None of the erase techniques disclosed in the following patents entirely fulfill this need. U.S. Pat. No. 4,584,482, issued Apr. 22, 1986, inventors Suzuki and Horikawa; U.S. Pat. No. 4,687,937, issued Aug. 18, 1987, inventors Aagano and Takasaki; U.S. Pat. No. 4,873,441, issued Oct. 10, 1989, inventors Kimura and Watanabe; U.S. Pat. No. 4,900,927, issued Feb. 13, 1990, inventors Kimura and Watanabe; U.S. Pat. No. 4,952,806, issued Aug. 28, 1990, inventor Mori; U.S. Pat. No. 5,072,119, issued Dec. 10, 1991, inventor Yamagichi.